U.S. patent application number 16/865342 was filed with the patent office on 2020-11-05 for method and apparatus for testing breast implants.
The applicant listed for this patent is Dwight D. Back, Harold J. Brandon. Invention is credited to Dwight D. Back, Harold J. Brandon.
Application Number | 20200345477 16/865342 |
Document ID | / |
Family ID | 1000004828906 |
Filed Date | 2020-11-05 |
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United States Patent
Application |
20200345477 |
Kind Code |
A1 |
Brandon; Harold J. ; et
al. |
November 5, 2020 |
Method and Apparatus for Testing Breast Implants
Abstract
This invention provides a method for determining breast implant
geometric properties, engineering stresses, engineering strains and
engineering moduli; directly and quickly, using a load frame
apparatus. More generally the invention provides a method for
determining geometric properties and engineering mechanical
properties of any elastomeric device, using a load frame apparatus.
Engineering stress and engineering strain properties of breast
implants are critical to their safety and durability. The geometric
properties of breast implants undergoing compression also relates
to the shape stability of breast implants, which may also be
related to clinical outcomes such as capsular contracture and other
untoward outcomes involving a breast capsule, such as Anaplastic
Large Cell Lymphoma (ALCL), double capsule formation, seroma
formation and associated breast implant illness (BII).
Inventors: |
Brandon; Harold J.; (St.
Louis, MO) ; Back; Dwight D.; (Satellite Beach,
FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Brandon; Harold J.
Back; Dwight D. |
St. Louis
Satellite Beach |
MO
FL |
US
US |
|
|
Family ID: |
1000004828906 |
Appl. No.: |
16/865342 |
Filed: |
May 2, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62920560 |
May 3, 2019 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61F 2250/0014 20130101;
A61F 2230/0063 20130101; A61F 2/12 20130101; A61F 2240/008
20130101; A61F 2230/0004 20130101 |
International
Class: |
A61F 2/12 20060101
A61F002/12 |
Claims
1. A method for determining the geometric properties and
engineering mechanical properties of a breast implant using a load
frame apparatus, the load frame apparatus comprising an upper plate
and a lower plate, the breast implant having a breast implant
volume and breast implant shell thickness, the method comprising
the steps of: Loading the breast implant into said load frame
apparatus, wherein said breast implant is disposed between said
upper plate and said lower plate; Implementing a dynamic load frame
apparatus program wherein said upper plate and said lower plate
form an area of contact with said breast implant, wherein the
separation distance between said upper plate and said lower plate
is plate spacing, wherein a dynamic compressive load is applied to
said breast implant by a dynamic change in plate spacing, and
wherein said plate spacing has a rate of change that is crosshead
speed; Recording said dynamic compressive load and said plate
spacing at a sampling rate; Providing a quasi-equilibrium geometric
model for said breast implant that is entirely a function of said
plate spacing and said breast implant volume; Computing the breast
implant geometric properties from said quasi-equilibrium geometric
model; and Computing said engineering mechanical properties from
said breast implant geometric properties, said breast implant shell
thickness, and said dynamic compressive load.
2. The method of claim 1, wherein said dynamic load frame apparatus
program is selected from the group consisting of an increasing
compressive load, an oscillatory compressive load further
comprising a frequency, a decreasing compressive load, and
combinations thereof.
3. The method of claim 1, wherein said geometric model is a
composite of a flattened cylinder and outer-half of a torus.
4. The method of claim 1, wherein said engineering mechanical
properties comprise at least one of engineering stresses,
engineering strains or engineering moduli.
5. The method of claim 1, wherein said breast implant geometric
properties are breast implant diameter D, breast implant surface
area A, and breast implant contact diameter d with said upper plate
and said lower plate.
6. The method of claim 1, wherein said quasi-equilibrium geometric
model includes breast implant diameter D, breast implant surface
area A, said plate spacing H, and said breast implant volume V
given by D = H + 1 2 ( - .pi. 2 H + 1 6 .pi. V H + ( .pi. 2 4 - 8 3
) H 2 ) ##EQU00007## A = .pi. 2 D 2 + ( .pi. 2 2 + .pi. ) DH + ( 3
.pi. 2 - .pi. 2 2 ) H 2 ##EQU00007.2##
7. The method of claim 1, wherein said area of contact is
lubricated.
8. The method of claim 1, wherein said upper plate and said lower
plate have a surface roughness on the order of about 1 micrometer
or less.
9. The method of claim 1, wherein the said upper plate and said
lower plate have surfaces that are parallel.
10. The method of claim 1, wherein said sampling rate ranges from
about 1 to about 1000 samples per second.
11. The method of claim 1, wherein said crosshead speed ranges from
about 0.005 cm/min to about 100 cm/min.
12. The method of claim 1, wherein said breast implant is an
anatomically shaped breast implant.
13. The method of claim 2, wherein said frequency ranges from about
1 Hz to 10 Hz.
14. The method of claim 4, wherein said engineering stresses are
selected from the group consisting of planform, circumferential and
normal.
15. The method of claim 4, wherein said engineering strains are
selected from the group consisting of projection, diametric and
areal.
16. A method for determining the geometric properties and
engineering mechanical properties of a multi-lumen breast implant
using a load frame apparatus, the load frame apparatus comprising
an upper plate and a lower plate, the multi-lumen breast implant
comprising at least two shell-enclosed lumens wherein each of the
at least two shell-enclosed lumens has a volume and shell
thickness, the method comprising the steps of: Loading the breast
implant into said load frame apparatus, wherein said multi-lumen
breast implant is disposed between said upper plate and said lower
plate; Implementing a dynamic load frame apparatus program wherein
said upper plate and said lower plate form an area of contact with
said multi-lumen breast implant, wherein the separation distance
between said upper plate and said lower plate is plate spacing,
wherein a dynamic compressive load is applied to said multi-lumen
breast implant by a dynamic change in plate spacing, and wherein
said plate spacing has a rate of change that is a crosshead speed;
Recording said dynamic compressive load and plate spacing at a
sampling rate; Providing a quasi-equilibrium geometric model for
said multi-lumen breast implant that is a function of said plate
spacing; Computing the multi-lumen breast implant geometric
properties from said quasi-equilibrium geometric model; and
Computing said engineering mechanical properties from said
multi-lumen breast implant geometric properties.
17. The method of claim 16, wherein said dynamic load frame
apparatus program is selected from the group consisting of an
increasing compressive load, an oscillatory compressive load
further comprising a frequency, a decreasing compressive load, and
combinations thereof.
18. The method of claim 16, wherein said area of contact is
lubricated.
19. The method of claim 16, wherein said quasi-equilibrium
geometric model further comprises separate quasi-equilibrium
geometric models for each of the at least two shell-enclosed
lumens.
20. A method for determining the geometric properties and
engineering mechanical properties of an elastomeric device using a
load frame apparatus, the load frame apparatus comprising an upper
plate and a lower plate, the elastomeric device having an
elastomeric device volume, the method comprising the steps of:
Loading the elastomeric device into said load frame apparatus,
wherein said elastomeric device is disposed between said upper
plate and said lower plate; Implementing a dynamic load frame
apparatus program wherein said upper plate and said lower plate
form an area of contact with said elastomeric device, wherein the
separation distance between said upper plate and said lower plate
is plate spacing, wherein a dynamic compressive load is applied to
said elastomeric device by a dynamic change in plate spacing, and
wherein said plate spacing has a rate of change that is crosshead
speed; Recording said dynamic compressive load and said plate
spacing at a sampling rate; Providing a quasi-equilibrium geometric
model for said elastomeric device that is a function of said plate
spacing and said elastomeric device volume; Computing the
elastomeric device geometric properties from said quasi-equilibrium
geometric model; and Computing said engineering mechanical
properties from said elastomeric device geometric properties and
said dynamic compressive load.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Provisional patent application 62/920,560, having a filing
or 371(c) date of May 3, 2019.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The field of this invention relates generally to test
methods employed to characterize the geometry and engineering
mechanical properties of breast prostheses, or breast implants, and
more generally, any elastomeric device. More specifically, the
field of this invention relates to engineering mechanical
properties, including, but not limited to, engineering stresses,
engineering strains, engineering moduli, pressure, and ultimate
strength of breast implants when a breast implant is subjected to
compressive forces. And, more specifically, this invention relates
to a quick and simplified method for determining breast implant
geometric and engineering mechanical properties using only
load-displacement data commonly measured using a load frame
apparatus, along with the breast implant's volume and shell
thickness.
Description of Related Art
[0003] Breast implants are regulated medical devices. Regulatory
bodies such as the FDA require testing of these medical devices to
prove that they are safe and durable.
[0004] Saline-filled and silicone gel-filled breast implants that
are marketed today have undergone extensive mechanical testing to
demonstrate strength and durability. Breast implant manufacturers
have followed the recommendations provided by medical device
regulatory bodies which require specific types of mechanical tests
and expected strength and durability requirements of silicone
shells and the finished product. Many of the current mechanical
evaluations utilize ASTM and ISO standards. Mechanical testing that
replicates clinical conditions is the goal of these experimental
procedures, but this is difficult to achieve in the laboratory.
Instead, breast implant manufacturers have used conventional
experimental techniques that can be performed in any well-equipped
mechanical testing laboratory.
[0005] Current routine breast implant testing includes tensile
strength, ultimate elongation, tear resistance, joint testing,
cyclic fatigue testing, ultimate strength tests, and valve
competency (for saline-filled breast implants). These mechanical
tests have not exactly simulated the in vivo environment. A test
methodology that simulates in vivo conditions would be the desired
procedure to evaluate the geometric and engineering mechanical
properties of breast implants, but this is difficult to achieve in
the laboratory.
[0006] Compression testing between two parallel plates, or platens,
in air or saline solution, has been used previously by breast
implant manufacturers for cyclic fatigue and ultimate strength
testing. Flat plate compression testing has been used to provide
breast implant cyclic fatigue failure characteristics, data for
fatigue lifetime predictions, ultimate strength, and morphological
features of fatigue failure.
[0007] To evaluate the stress and strain properties of breast
implants when subjected to a compressive load, current methods
entail complex and time-intensive finite element or finite
difference modeling, and are not readily applied to assess the
safety and durability of breast implants.
[0008] Hence, it is highly desirable to have a simplified method
for evaluating the geometric and engineering mechanical properties
of breast implants. This simplified method is needed not only to
assess the safety and durability of current breast implants, but
also to facilitate the design of safer and more durable future
breast implants.
[0009] The geometric and engineering mechanical properties of
breast implants subjected to compressive forces may also relate
clinical outcomes such as capsular contracture, and other untoward
outcomes involving a breast capsule, such as Anaplastic Large Cell
Lymphoma (ALCL), double capsule formation, seroma formation and
associated breast implant illness (BII), and in vivo rupture.
Consequently, a simplified and easily implemented means to assess
these breast implant properties could aid in understanding their
relationship to current clinical outcomes, and in designing breast
implants which will have better clinical outcomes.
OBJECTS OF THE INVENTION
[0010] This invention has several objects to address the
deficiencies of current breast implant testing methods:
[0011] An object of this invention is to provide a simplified and
quick means to determine breast implant geometries, and changes in
geometry, entirely from common load-displacement data measured
using a load frame apparatus and the breast implant's volume and
shell thickness.
[0012] An object of this invention is to provide the means to
determine the various engineering mechanical properties (e.g.,
engineering stresses, engineering strains, and engineering moduli)
of breast implants entirely from common load-displacement data
measured using a load frame apparatus and the breast implant's
volume and shell thickness.
[0013] Another object of this invention is to provide a simplified
method that uses a quasi-equilibrium assumption for an implant's
geometric state, which allows a breast implant's geometric
properties and engineering mechanical properties to be determined
by a load frame apparatus implementing a dynamic test program. This
eliminates the need for time-intensive static load frame apparatus
testing, whereby a load would have to be stepwise applied, followed
at each step with manual measurements of an implant's geometry.
[0014] Another object of this invention is to provide the breast
implant industry a means to assess the impact of breast implant
design changes, including but not limited to, filler material,
shell thickness, breast implant fill volume, and breast implant
shape, on the safety and durability of a breast implant.
[0015] Yet another object of this invention is to provide the means
to comprehensively evaluate the geometric and engineering
mechanical properties of breast implants during ultimate strength
testing and cyclic fatigue testing using load and displacement data
from a load frame apparatus.
[0016] Another object of this invention is to provide breast
implant manufacturers a means to quickly determine a comprehensive
series of breast implant properties during product design, product
development, production, or quality assurance.
[0017] Yet another object of this invention is to provide a method
to characterize the geometry and engineering mechanical properties
of a variety of breast implant designs including, but not limited
to, multi-lumen breast implants and anatomically shaped breast
implants.
[0018] An object of this invention is also to provide a simplified
and quick means to determine geometric and engineering mechanical
properties of elastomeric devices other than breast implants.
BRIEF SUMMARY OF THE INVENTION
[0019] This invention relates to quickly determining the geometry
and engineering mechanical properties (e.g., engineering stresses,
engineering strains and engineering moduli) of breast implants, in
their implanted or implantable form, using only the
load-displacement data that is typically generated from a load
frame apparatus during mechanical testing procedures. When
referring to a breast implant in this invention disclosure, it is
understood that the breast implant is in its implanted or
implantable form. A breast implant will comprise at least one
shell-enclosed lumen, and breast implants comprised of more than
one shell-enclosed lumen may also be referred to as a multi-lumen
breast implant.
[0020] It was discovered that a geometry state of quasi-equilibrium
could be assumed when breast implants are dynamically tested in a
load frame apparatus, thereby eliminating the need for
time-intensive manual measurements and data recording during load
frame testing after each stepwise load change is applied to a
breast implant. A quasi-equilibrium process is one in which the
deviation from equilibrium is infinitesimal, and all the states the
breast implant passes through during the transient, or dynamic,
process may be considered equilibrium states. Therefore, this
assumption constrains all breast implant properties to be constants
for every instantaneous compression load during the automatic, or
dynamic, load-displacement test process or dynamic load
program.
[0021] This invention simplifies the determination of crucial
breast implant properties that relate directly to the safety and
durability of breast implants or other elastomeric devices. The
invention also provides a quick test, taking only minutes, to
ascertain a broad spectrum of geometric and engineering mechanical
properties of a breast implant.
[0022] Using this invention, load (or force) measurements are
acquired as a function of platen displacement, or plate spacing, in
a load frame apparatus. These measured load-displacement parameters
can be acquired automatically with a data acquisition system or
recorded manually. With this load-displacement data, all geometric
properties and engineering mechanical properties of the breast
implant can be calculated knowing the breast implant volume and
shell thickness if the breast implant is comprised of a shell
enclosing a fluid such a saline solution or elastomeric filler such
as a silicone gel.
[0023] The examples and description in this disclosure primarily
reference a breast implant, however, it is understood that this
invention can also be applied to other elastomeric devices for
which the geometry of the elastomeric device can be accurately
modeled in terms of geometric shapes or composites thereof that can
be derived mathematically using plate spacing data from a load
frame apparatus and the device's volume. An elastomeric device is
understood to be any device comprised of an elastomeric material
that encloses a lumen filled with a fluid or gel. In the context of
this invention and engineering terminology, the term fluid is
understood to be any gaseous or liquid material. An elastomeric
device may also be monolithic or contiguous, with no enclosed
fluids or gels.
[0024] This invention has been shown to produce accurate modeling
of breast implant geometry, and therefore accurate engineering
mechanical properties (i.e., stresses, strains and moduli) that are
a function of the measured load and breast implant geometry.
[0025] This invention provides the means to quickly determine
breast implant geometry using only the plate spacing data and
breast implant volume, utilizing the novel approach of modeling a
compressed breast implant as a combination of a flattened cylinder
and the outer-half of a torus. The area of contact of the load
frame platens with the breast implant can also be determined by
this invention.
[0026] Given the geometric properties of a breast implant
undergoing compression, various engineering mechanical properties
can then be determined from the measured load profile. In
particular, the internal pressure of the breast implant can be
calculated from the load and platen contact area, the
circumferential stress can be calculated from the load, shell
thickness and breast implant geometry, and multi-dimensional
strains can be calculated using the breast implant geometry.
[0027] Engineering stresses that can be determined by this
invention include, but are not limited to, planform,
circumferential and normal.
[0028] Engineering strains that can be determined using this
invention include, but are not limited to, projection, diametric
and areal, which refer to changes in breast implant height,
diameter and surface area, respectively. Collectively, these
engineering strains may be referred to as multi-dimensional
strains. The breast implant multi-dimensional strains and
associated breast implant shape change due to compression is a
mechanical property that has not been previously evaluated for
breast implants. Breast implant strain is directly related to
breast implant shape change.
[0029] Engineering moduli, or the ratio of stress to strain, can
also be calculated by this invention for a breast implant from only
the load and displacement data and knowing the breast implant
volume and shell thickness. Engineering moduli are important
properties of breast implants which describe the compressibility,
firmness and shape stability of a breast implant, which are crucial
properties related to the safety and durability of breast
implants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 summarizes the process or protocol steps for
determining engineering mechanical properties (i.e., engineering
stresses, engineering strains and engineering moduli) using a load
frame apparatus.
[0031] FIG. 2 illustrates the geometric descriptors for a breast
implant undergoing compression in a load frame apparatus.
[0032] FIG. 3 illustrates the geometric components comprising the
flattened cylinder and outer-half of a torus composite geometric
model for a compressed breast implant (cross section view).
[0033] FIG. 4 compares load (F)-plate spacing (H) for manual
(static) measurements to automatic (dynamic) compression testing of
two breast implants.
[0034] FIG. 5 compares the automatic (dynamic) breast implant
diameter to manually (statically) measured breast implant diameter
for two breast implants undergoing compression in a load frame
apparatus.
[0035] FIG. 6 illustrates the computed surface areas for two breast
implants undergoing dynamic compression in a load frame
apparatus.
[0036] FIG. 7 illustrates the diametric strain for three breast
implants as a function of load.
[0037] FIG. 8 illustrates stress vs. strain curves for two breast
implants generated using the methods of this invention.
[0038] FIG. 9 illustrates engineering moduli for three breast
implants calculated using the method of this invention.
BRIEF DESCRIPTION OF SYMBOLS & NUMBERS
[0039] 11--Process or protocol step of calibrating the load frame
apparatus. [0040] 12--Process or protocol step of optionally
lubricating the load frame plates, optionally lubricating the
breast implant, or optionally lubricating both. [0041] 13--Process
or protocol step of loading a breast implant between two platens
comprising the load frame apparatus. [0042] 15--Process or protocol
step of adjusting the spacing between the plates of the load frame
apparatus so that the load on the breast implant is near zero. This
is the initial state before commencing the testing process of this
invention. [0043] 17--Process or protocol step of implementing a
dynamic load program with the load frame apparatus. [0044]
21--Process or protocol step of manufacturing a breast implant to
be tested by the method of this invention. [0045] 23--Process or
protocol step of recording the breast implant volume, shell
thickness, and initial breast implant geometry. [0046] 25--Process
or protocol step of recording the plate spacing and load applied to
an implant in its initial near-zero load state. [0047] 27--Process
or protocol step of recording the plate spacing and applied load to
a breast implant while the dynamic load program is being
implemented by the load frame apparatus. [0048] 30--Process or
protocol step of computing the geometry and engineering mechanical
properties (i.e., engineering stresses, engineering strains and
engineering moduli) of a breast implant subjected to dynamic
compressive load using only load frame apparatus dynamic plate
spacing, load frame apparatus dynamic load, breast implant volume
and breast implant shell thickness. [0049] 100--Breast implant or
other elastomeric device (cross section view). [0050]
101--Flattened cylinder component of composite geometric model for
a compressed breast implant (cross section view). [0051]
102--Outer-half of torus component of composite geometric model for
a compressed breast implant (cross section view). [0052] 201--Upper
platen, or plate, of a load frame apparatus. [0053] 202--Lower
platen, or plate, of a load frame apparatus.
[0054] A--Surface area of a breast implant or elastomeric device.
A.sub.o refers to the initial implant surface area at near-zero
load or before a load has been applied.
[0055] A.sub.x--Area "x" used for computing a stress. (Example:
A.sub.p is the planform area .pi.D.sup.2/4).
[0056] B.I.--Breast implant.
[0057] D--Diameter of a breast implant or elastomeric device.
D.sub.o refers to the initial implant diameter at near-zero load or
before a compressive load has been applied.
[0058] d--Contact diameter of a breast implant or elastomeric
device with the upper and lower platens of a load frame
apparatus.
[0059] e.sub.g--Engineering strain based on geometric property
g.
[0060] E.sub.xg--Engineering modulus based on stress S.sub.x and
geometric property g.
[0061] F--Force or load. In the context of this invention, the
force or load is compressive to the elastomeric device or breast
implant being tested.
[0062] g--Generalized geometric property of breast implant or
elastomeric device, e.g., H, D and A.
[0063] g.sub.o--Geometric property at near-zero or minimal
compressive load.
[0064] H--Plate spacing, or distance between the upper and lower
platens, or plates, of a load frame apparatus. Also may be referred
to as platen displacement, or simply displacement. This also refers
to the breast implant height or elastomeric device height, or
breast implant projection or elastomeric device projection, in the
load frame apparatus during load frame testing. H.sub.o refers to
the initial breast implant or elastomeric device height, or
projection, at near-zero load or before a load has been
applied.
[0065] M--Mass of breast implant.
[0066] P--Pressure.
[0067] .rho.--Density of breast implant.
[0068] S.sub.x--Engineering stress based on area A.
[0069] t--Shell thickness of a breast implant or elastomeric device
comprised of a shell enclosing a fluid or gel.
[0070] V--Volume of breast implant or elastomeric device. When this
invention is applied to a breast implant, volume V can represent
the total volume of the elastomeric device or breast implant
including all shell-enclosed lumens comprising the elastomeric
device or breast implant if the elastomeric device or breast
implant is a multi-lumen elastomeric device or breast implant. This
invention can also be applied to the individual shell-enclosed
lumens of the multi-lumen elastomeric device or breast implant, in
which case there can be a volume V for each of the shell-enclosed
lumens of the multi-lumen elastomeric device or breast implant.
DETAILED DESCRIPTION OF THE INVENTION
[0071] This invention relates to quickly determining the geometry
and engineering mechanical properties of breast implants undergoing
dynamic compression using only load-displacement data that are
typically measured and recorded by a load frame apparatus. Breast
implants are typically comprised of one or more lumens enclosed by
a shell, with the one or more lumens filled with a fluid or gel.
This invention simplifies the determination of crucial breast
implant geometric properties and engineering mechanical properties
that relate directly to the safety and durability of breast
implants.
[0072] A load frame apparatus typically comprises an upper plate
and a lower plate that have surfaces that are parallel to one
another, whereby at least one of the platens, or plates, is
controllably movable relative to the other thereby controlling the
plate spacing and compression on a device disposed between the
upper plate and lower plate. The separation distance between the
two platens, or plates, is referred to as the plate spacing, or
displacement, (H). The plate spacing can be programmably controlled
to continuously (i.e., dynamically) or discretely (i.e., stepwise)
decrease or increase with a crosshead speed, thus increasing or
decreasing the compressive load on the object placed between the
upper and lower platens. Discretely controlled plate spacing allows
for static or manual measurements of breast implant geometric
properties and plate spacing as it is subjected to a
stepwise-applied load, whereas continuously or dynamically
controlled plate spacing provides for dynamic measurements and
automatic recording of breast implant geometric properties as it is
subjected to a dynamic compressive load.
[0073] In a preferred embodiment, this invention provides a method
for utilizing dynamic measurements and appropriate geometry models
of the compressed breast implant to quickly determine geometric
properties and engineering mechanical properties of a breast
implant without the need for complex and time-intensive
calculations such as finite element or finite difference modeling,
or time-intensive manual (or static) measurements.
[0074] The dynamic load program may comprise an increasing
compressive load (decreasing plate spacing), an oscillatory
compressive load having a frequency, a decreasing compressive load
(increasing plate spacing), or any combinational sequence thereof.
A load cell integrated into the load frame apparatus provides
accurate measurement of force F, or load, exerted on an object
disposed between the upper and lower platens, or plates.
State-of-the-art load frames will allow plate spacing and load
measurements to be recorded digitally as a function of time
according to a prescribed data acquisition sampling rate.
[0075] FIG. 1 summarizes the process or protocol steps comprising
the test method of this invention whereby a load frame apparatus is
used to compute the engineering mechanical properties (i.e.,
engineering stresses, engineering strains and engineering moduli)
of a breast implant. The terminology such as crosshead speed, load
frame, load, displacement, platen spacing, and platens (or plates)
used to describe the invention's method of FIG. 1 and throughout
this invention disclosure is terminology known by, and used by,
those skilled in the art of mechanical testing. The load frame is
first calibrated 11. A breast implant is manufactured 21 and is
loaded into the load frame apparatus and disposed between the two
plates 13 of a calibrated load frame. Preferably, and prior to
loading the breast implant between the platens, the breast implant
volume V, shell thickness t, and initial implant geometry (e.g.,
D.sub.o, A.sub.o, and H.sub.o), should be recorded 23. However, the
volume V and shell thickness t may also be recorded after
completion of load program 17 when the breast implant has been
removed from the load frame apparatus after completion of the test.
The plate spacing H is then adjusted to a starting point 15, or
initial state, for the testing so that both plates form an area of
contact with the breast implant with minimal or near-zero load
imparted on the breast implant. The plate spacing and near-zero
initial compressive load is recorded 25. This type of initial state
adjustment is commonly practiced by those skilled in the art of
mechanical testing. The interface, or area of contact, between the
platens and the breast implant, may be first optionally lubricated
with a layer of lubricant as indicated by 12. To lubricate the
interface, lubricant may be applied to the breast implant, load
frame apparatus platens, or both. The lubricant must be compatible
with the material of the breast implant to avoid alteration of the
material properties of the breast implant during the test. Although
the lubrication step of 12 is optional, it is a preferred
embodiment of this invention to test breast implants both with
lubrication and without lubrication. Examples of lubricants
include, but are not limited to, aqueous solutions and silicone
oils. A dynamic load program is then implemented 17 on the breast
implant, whereby a compressive load is applied dynamically to the
breast implant by dynamically changing the displacement (or plate
spacing) between the upper plate and lower plate. The rate of
change in displacement (or plate spacing) between the upper plate
and lower plate in the field of mechanical testing is characterized
as having a crosshead speed. In most load frame apparatus, this is
equivalent to the rate or speed at which the upper plate moves
relative to a stationary lower plate. The dynamically changing
plate spacing (or displacement) and load are then recorded 27
during implementation of the dynamic load program 17. The dynamic
plate spacing and dynamic compressive load are preferably
automatically recorded using a data acquisition system integrated
with the load frame apparatus. The implemented load program 17 may
comprise an increasing compressive load (decreasing plate spacing),
an oscillatory compressive load, a decreasing compressive load, or
any combinational sequence thereof. The plate spacing H and load F
are preferably recorded automatically by a data acquisition and
storage system at a sampling rate, and then used along with the
known constant breast implant's volume V and shell thickness t to
compute the breast implant's geometry (e.g., contact diameter d of
breast implant with the upper and lower plates, breast implant
diameter D, and breast implant surface area A) and engineering
mechanical properties (i.e., engineering stresses, engineering
strains and engineering moduli) of the breast implant 30. Software
may also be integrated into the data acquisition system of the load
frame apparatus which provides a quasi-equilibrium geometric model
for the implant, and converts the load-displacement raw data to
implant geometry and engineering mechanical properties for digital
or graphical display.
[0076] Using this invention, the load F (or force) measurements are
acquired as a function of breast implant height or plate spacing H
in a load frame apparatus as the plate spacing is dynamically
changed. Breast implants generally have a height of about 10
centimeters (cm) or less; and with preferred load frame crosshead
speeds of about 2.5 centimeters/minute (cm/min) to about 50 cm/min,
the time to perform a test run can be very quick. With the load F
and plate spacing H data, all breast implant geometric properties
and engineering mechanical properties can be calculated knowing the
constant breast implant volume V and shell thickness t of the
breast implant shell, or enclosure. The breast implant volume V is
typically provided by the breast implant manufacturer; however, a
breast implant's volume V can be easily determined from the breast
implant's mass M and the density p of the material(s) comprising
the breast implant using the formula V=M/p. Since the method of
this invention may only take a few minutes, and calculations from
the data can be automated by computer program or spreadsheet, a
full description of breast implant geometry and engineering
mechanical properties can be obtained very quickly.
[0077] FIG. 2 is a drawing of a breast implant undergoing
compression in a load frame apparatus. The upper platen 201 and
lower platen 202 of a load frame apparatus contact the breast
implant 100 with a diameter d. The breast implant also has a
diameter D, which is the distance between the outer edges of the
breast implant, and the distance between the upper platen 201 and
lower platen 202 is H.
[0078] Referring to FIG. 3, it was discovered that a composite
geometric model for a flattened cylinder 101 and the outer-half of
a torus 102 accurately represents the geometry of a compressed
breast implant. FIG. 3 also shows the dimensions d, D, and H which
describe the geometry of the flattened cylinder and torus
components. The volume V of this composite geometric model is given
by:
V = ( volume cylinder ) + ( volume outer - half of torus ) , V =
.pi. d 2 H 4 + 2 .pi. ( 2 H 3 .pi. + d 2 ) .pi. H 2 8 ( 1 )
##EQU00001##
[0079] where V is the breast implant volume, H is the plate spacing
or breast implant height, and d is the diameter of contact of the
breast implant with the upper platen 201 and lower platen 202. The
volume V of Equation 1 can be the total breast implant volume,
including the volume of all shell-enclosed lumens if the breast
implant is comprised of multi-lumens. For a multi-lumen breast
implant, an equation such as Equation 1 may also represent the
shell-enclosed lumen volume of each individual shell-enclosed lumen
of the multi-lumen breast implant if each shell-enclosed lumen of
the multi-lumen breast implant are to be analyzed separately using
the method of this invention.
[0080] Equation 1 can be solved for the breast implant-platen
contact diameter d as
d = 1 2 ( - .pi. H 2 + 1 6 .pi. V H + ( .pi. 2 4 - 8 3 ) H 2 ) ( 2
) ##EQU00002##
[0081] Further, it was discovered that the outer curved perimeter
surface of a breast implant undergoing compression can be
accurately represented geometrically as a semi-circle having a
diameter equal to H, which also corresponds to the diameter of the
outer half of the torus. Equation 2 for breast implant-platen
contact diameter d can then be simplified to an equation for the
breast implant diameter D that is entirely a function of plate
spacing H and the known constant breast implant volume V:
D = H + d ( 3 ) D = H + 1 2 ( - .pi. 2 H + 1 6 .pi. V H + ( .pi. 2
4 - 8 3 ) H 2 ) ( 4 ) ##EQU00003##
[0082] The same geometric assumption can also be used by this
method to accurately calculate the surface area of a breast
implant. Since it was discovered that a compressed breast implant
can be accurately modeled as a composite of a flattened cylinder
and outer-half of a torus, the surface area is:
A = ( area cylinder faces ) + ( area outer - half of torus ) , A =
.pi. d 2 2 + .pi. 2 d H 2 + .pi. H 2 , A = .pi. 2 D 2 + ( .pi. 2 2
+ .pi. ) DH + ( 3 .pi. 2 - .pi. 2 2 ) H 2 ( 5 ) ##EQU00004##
[0083] Where A is the surface area of the breast implant. Since D
is a function of plate spacing H and implant volume V by Equation
4, the surface area A can be calculated entirely from the load
(F)-plate spacing (H) data and the known breast implant volume
V.
[0084] Hence, using this invention's geometric model as a
quasi-equilibrium state for a breast implant subjected to a dynamic
compressive load, the breast implant geometry (e.g., diameter D,
diameter d, and surface area A) can be computed entirely from the
dynamic plate spacing H data and the known constant breast implant
volume V.
[0085] Engineering stresses can also be calculated from the
measured load F, shell thickness t and breast implant geometry,
where the breast implant geometry is directly derived from the
dynamic load frame apparatus load-displacement data and a
quasi-equilibrium assumption for implant geometry as described in
the preceding paragraphs and Equations 1-5. Engineering stresses
are forces divided by an area. For a breast implant, the force is
the applied compressive load, and various areas (A.sub.X) can be
used including, but not limited to, .pi.d.sup.2/4, .pi.d.sup.2/4,
and .pi.Dt. The general equation for computing various engineering
stress by this method is:
S.sub.x=Stress=F/A.sub.x (6)
[0086] Where S.sub.x denotes a stress S based on area A.sub.x. The
area A.sub.x used to compute a stress can be the planform area
.pi.D.sup.2/4, shell cross section area .pi.Dt, platen contact area
.pi.d.sup.2/4, or some other area. Other more complex formulas for
stress, depending on whether the compressed breast implant is best
described as thin-walled or thick-walled are known by those skilled
in the art, and are functions of geometric properties of the breast
implant. For example, the maximum stress at the outer perimeter of
a compressed breast implant can be derived as:
Stress = F ( D 2 - d 2 ) .pi. tDd 2 ( 7 ) ##EQU00005##
[0087] This stress can also be calculated using only load
(F)-displacement (H) data, since d and D are both functions of H
per Equations 2 and 4, and the known and easily measured breast
implant shell thickness t.
[0088] The breast implant internal pressure may be defined as a
normal stress, whereby the area A.sub.x is the breast
implant-platen contact area .pi.d.sup.2/4:
P=Pressure=F/(.pi.d.sup.2/4) (8)
[0089] Engineering strain can be calculated using the breast
implant geometry, which is entirely a function of the plate spacing
H and known implant volume V using the method of this invention.
Engineering strain is defined by the change is a geometric property
relative to the initial geometric property value
(.DELTA.g=g-g.sub.o) divided by the initial geometric property
g.sub.o, where the geometric property g includes, but is not
limited to, breast implant diameter D, plate spacing or breast
implant height H, and breast implant surface area A. The general
equation for engineering strain is:
Strain = e g = g - g o g o ( 9 ) ##EQU00006##
[0090] Where g.sub.o is the initial geometric property g of the
breast implant prior to applying a compressive load F. The strain
associated with the height of the breast implant, diameter of the
breast implant, and surface area of a breast implant are also
referred to as projection, diametric and areal strain,
respectively. This invention provides the means to determine the
geometric properties g from the plate spacing measurements acquired
using a load frame apparatus.
[0091] This method also provides the means to determine engineering
moduli from load-displacement data. Engineering moduli are
generally defined as the ratio of a stress to a strain.
Modulus=E.sub.xg=.DELTA.S.sub.x/.DELTA.e.sub.g (10)
[0092] Where .DELTA.S.sub.x is the change in stress based on area
A.sub.x and .DELTA.e.sub.g is the change in strain for geometric
property g over a defined segment of the stress-strain curve.
Typically, an engineering modulus is defined for the linear portion
of the stress-strain curve where S.sub.x and e.sub.g approach zero.
However, a tangent modulus can also be calculated over any portion
of the stress-strain curve as the slope of the curve.
First Invention Embodiment
[0093] FIG. 4 illustrates load (F)-displacement (H) measurements
for two breast implants. The load frame crosshead speed used for
the testing of FIG. 4 was 25.4 cm/min, making the test run time for
the data shown in FIG. 4 on the order of a few minutes. In FIG. 4,
Breast implant #1 is a saline-filled dual-lumen breast implant in
which the breast implant-platen area of contact is not lubricated,
and Breast implant #2 is a single-lumen gel-filled breast implant
in which the breast implant-platen area of contact is not
lubricated.
[0094] These load-displacement measurements can be used to
calculate all other breast implant geometric properties as well as
engineering stresses, engineering strains and engineering moduli.
FIG. 4 also illustrates that the load (F)-plate spacing (H)
measurements may be taken manually or automatically by a data
acquisition system integrated with the load frame apparatus. The
manual (e.g., static) and automatic (e.g., dynamic) measurements on
the same breast implant in FIG. 4 also illustrate an important
novel feature of this invention, whereby an assumption of
quasi-equilibrium can be applied accurately when measuring the load
F and geometry of a compressed breast implant. This assumption of
quasi-equilibrium is validated as shown in FIG. 4 by the close
agreement between the manual, or static, measurements, whereby a
load is applied in a stepwise and discrete method, allowing a
static state of equilibrium to be achieved between each load F and
plate spacing H measurement, and the automatic, or dynamic,
measurements, whereby a load F is continuously or dynamically
applied while recording the plate spacing H. A quasi-equilibrium
process is one in which the deviation from equilibrium is
infinitesimal, and all the states the breast implant passes through
during the transient process may be considered equilibrium states.
Therefore, the quasi-equilibrium assumption approximates all breast
implant properties to be constants for every instantaneous
compression load during the dynamic, or automatic,
load-displacement test process or load program. These breast
implant properties include the breast implant diameter D, breast
implant H, breast implant surface area A, contact diameter d of the
breast implant with the upper and lower plates, contact area of the
breast implant with the upper and lower plates, and overall breast
implant shape.
Second Invention Embodiment
[0095] FIG. 5 also illustrates how this invention can be accurately
applied to calculate the breast implant diameter D from Equation 4
as a function of load F using the plate spacing H data generated
while implementing a dynamic load program. FIG. 5 also illustrates
that a quasi-equilibrium assumption is valid, and that Equation 4
accurately represents the geometry of a compressed breast implant
by comparing the manually measured breast implant diameter D to the
computed breast implant diameter D. The manual, or static,
measurements of D are taken at discrete loads, whereby the load
frame is paused and manual measurements for D are taken. The
automatic, or dynamic, values for D are determined from the dynamic
data for plate spacing with a geometric model and quasi-equilibrium
assumption as in Equation 4. In FIG. 5, Breast implant #1 is a
gel-filled single-lumen breast implant in which the breast
implant-platen area of contact is not lubricated, and Breast
implant #2 is a another single-lumen gel-filled breast implant in
which the breast implant-platen area of contact is not
lubricated.
[0096] FIG. 6 further illustrates that this invention can be used
to determine the breast implant surface area A, which is a function
of the plate spacing H (measured dynamically during implementation
of a load program) when substituting Equation 4 for the breast
implant diameter D into Equation 5. In FIG. 6, Breast implant #1 is
a saline-filled dual-lumen breast implant in which the breast
implant-platen area of contact is not lubricated, and Breast
implant #2 is a single-lumen gel-filled breast implant in which the
breast implant-platen area of contact is not lubricated.
Third Invention Embodiment
[0097] Using only the load (F)-displacement (H) data that is
automatically sampled and recorded from a load frame apparatus
while implementing a dynamic load program, various engineering
strains can be calculated. FIG. 7 illustrates the diametric strain
calculated from Equation 9 and dynamic load frame measurements,
where the geometric property g is the breast implant diameter D, as
a function of load F. The strain in FIG. 7 is expressed as a
percentage, which is Equation 9 multiplied by 100%. Similarly,
other strains for breast implant height H and breast implant
surface area A can be computed. Given a breast implant's volume V
(a constant), all strains can therefore be computed using the plate
spacing H of the load frame apparatus through Equations 4 and 5. In
FIG. 7, Breast implant #1 is a dual-lumen saline-filled breast
implant in which the breast implant-platen area of contact is
lubricated, Breast implant #2 is a gel-filled single-lumen breast
implant in which the breast implant-platen area of contact is
lubricated, and Breast implant #3 is another gel-filled
single-lumen breast implant in which the breast implant-platen area
of contact is lubricated.
Fourth Invention Embodiment
[0098] FIG. 8 illustrates that engineering stresses can be
calculated using the load (F)-displacement (H) data that is
automatically sampled and recorded from a load frame apparatus
while implementing a dynamic load program. The stress shown in FIG.
8 is the planform stress which is defined as the load divided by
the planform area A.sub.p=.pi.D.sup.2/4, where D is the breast
implant diameter. This corresponds to Equation 6, whereby the
planform area A.sub.p is used for A. Other stresses, including but
not limited to, circumferential and normal stress, can also be
calculated by this invention using the load F and plate spacing H
data. In FIG. 8, Breast implant #1 is a dual-lumen saline-filled
breast implant in which the breast implant-platen area of contact
is lubricated, and Breast implant #2 is a gel-filled single-lumen
breast implant in which the breast implant-platen area of contact
is lubricated.
Fifth Invention Embodiment
[0099] Engineering moduli can also be determined using this
invention. As shown in FIG. 9, an engineering modulus E.sub.pD
based on the planform area stress S.sub.p and diametric strain
e.sub.D is plotted. The diametric strain is expressed as a
percentage in FIG. 9, or Equation 9 multiplied by 100%. This
modulus is defined as a tangent modulus, since it is the slope of
the stress-strain curve. This particular modulus describes the
shape stability of a breast implant, or its resistance to changing
shape. The higher the modulus value, the higher the shape stability
under compression. Other more traditional moduli are defined as the
linear portion of a stress-strain curve as the stress and strain
approach zero. This invention provides the means to determine all
engineering moduli using only the load F and plate spacing H data
from a load frame apparatus given a breast implant's constant
volume V. In FIG. 9, Breast implant #1 is a dual-lumen
saline-filled breast implant in which the breast implant-platen
area of contact is lubricated, Breast implant #2 is a gel-filled
single-lumen breast implant in which the breast implant-platen area
of contact is lubricated, and Breast implant #3 is another
gel-filled single-lumen breast implant in which the breast
implant-platen area of contact is not lubricated.
Additional Embodiments
[0100] Although this invention is described in terms of breast
implants, this invention may also be applied to determine the
geometric and engineering mechanical properties of other
elastomeric devices undergoing compression. It is also contemplated
that this invention can be utilized during the design, development,
production, or quality testing of other devices comprised of a
monolithic or contiguous elastomeric matrix (i.e., devices that do
not have a distinct enclosing shell), or devices comprised of an
enclosing thin or thick-walled elastomeric shell filled with a
fluid or gel.
[0101] It is further contemplated that an elastomeric device may be
comprised of a multi-lumen structure of at least two shell-enclosed
lumens, whereby each of the at least two shell-enclosed lumens is
filled with a fluid or gel. For example, for dual-lumen breast
implant devices, the two shell-enclosed lumens are typically
"nested," with one shell-enclosed lumen contained within the lumen
of, and sharing a common axis with, a second shell-enclosed lumen.
There are breast implants comprised of dual lumen saline-filled
silicone shells, breast implants having two lumens enclosed by
shells having one lumen filled with silicone-gel and the other
saline, and breast implants having two lumens each enclosed by
silicone shells with one lumen filled with a fluid such as air and
the other lumen filled with silicone gel or saline. This invention
can be used to determine the geometric properties as well as the
engineering stresses, engineering strains and engineering moduli
for each of the shell-enclosed lumens comprising the multi-lumen
elastomeric device or multi-lumen breast implant using
quasi-equilibrium geometric models and engineering mechanical
models such as Equations 1-10, wherein each shell-enclosed lumen
can be characterized by a volume and shell thickness and separate
geometric models. This invention can also be used to determine the
overall geometric properties, engineering stresses, engineering
strains and engineering moduli of a multi-lumen elastomeric device
or breast implant when using the total volume (i.e., the volume of
all shell-enclosed lumens comprising the elastomeric device or
breast implant) of the multi-lumen elastomeric device or breast
implant with quasi-equilibrium geometric models such as Equations
1-10.
[0102] It is also contemplated that this invention can be used as
part of cyclic fatigue and ultimate strength testing of elastomeric
devices to provide the elastomeric device designer a thorough
understanding of the stresses and strains endured by the
elastomeric device. During cyclic fatigue testing, elastomeric
devices (e.g., breast implants) are cycled between two compressive
loads with a frequency, in which one of the compressive loads could
be zero or near-zero. Typically, cyclic fatigue load frequencies
used for fatigue testing ranges from about 1 Hz to about 10 Hz,
however this method could be used with any cyclic load frequency
achievable by the load frame apparatus.
[0103] A range of crosshead speeds may be used with this invention
since the quasi-equilibrium assumption coupled with an appropriate
geometric model for a compressed breast implant was discovered to
be valid. For automated, or dynamic, load frame testing, typical
crosshead speeds that can be used for this invention range from
about 0.005 cm/min to about 100 cm/min, with preferred crosshead
speeds ranging from about 2.5 cm/min to about 50 cm/min.
[0104] As a breast implant is dynamically compressed by a load
frame apparatus using the method of this invention, the compressive
load and plate spacing are recorded at a sampling rate programmed
by the data acquisition system integrated with the load frame
apparatus. Preferred sampling rates for this invention are about 1
to about 1000 samples per second.
[0105] The force, or load, range utilized by this invention for a
breast implant should correspond to those compressive forces
typically encountered in vivo. Typical in vivo forces on breast
implants can reach about 1000 N, although higher forces may be
possible during less common events such as automobile accidents or
falling from a height.
[0106] It is also contemplated that upper plate, lower plate or
both upper and lower plate can have a surface that is contoured
(for example, to simulate the curvature of a human chest wall in
the case of a breast implant.)
[0107] It is further contemplated that a software program may be
integrated into the load frame apparatus electronics and data
acquisition system to provide conversion of the load-displacement
raw data directly to breast implant geometric properties and
engineering mechanical properties using an appropriate
quasi-equilibrium geometric model for the compressed breast
implant.
[0108] When breast implants are inside the breast capsule, they are
surrounded by synovial fluid which can provide a varying degree of
lubrication at the breast implant-breast capsule interface. The
degree of lubrication within a breast capsule can also vary by a
person's specific physiology and the viscosity of the synovial
fluid surrounding the breast implant in a breast capsule.
Therefore, a preferred embodiment of this method is to test breast
implants both with and without a lubricated interface between the
breast implant and platens. Lubrication consists of applying a
layer of aqueous solution, silicone oil or other lubricating fluid
at the contact area interface between the load frame apparatus
platens and the breast implant. Breast implants can also be tested
using lubricants having a viscosity range expected to simulate the
viscosity range of synovial fluid in a breast capsule, or by using
synthetic synovial fluid. Testing breast implants with both
lubricated and unlubricated interfaces with the load frame
apparatus platens ensures that the engineering stresses, strains
and moduli are characterized over the full spectrum of possible in
vivo environments. In fact, during the development of this
invention, it was discovered that the engineering stresses,
strains, and moduli with lubricated and unlubricated breast
implant-platen interfaces may be different.
[0109] It is also contemplated that this invention can be used to
characterize the engineering mechanical properties of anatomically
shaped breast implants, since although anatomically shaped breast
implants are not symmetric in a zero-load state, these implants
will deform to a flattened cylinder and the outer-half of a torus
geometry when subjected to a compressive load between the two
platens of a load frame apparatus.
[0110] The preferred platens comprising the load frame apparatus
should be constructed of a rigid material including, but not
limited to, stainless steel, aluminum, and high strength plastics.
Preferably, the surface roughness for the platens should be in the
"polished" category, having a surface roughness on the order of
about 1 micrometer or less.
[0111] For implementation of this invention, it is also preferred
that the lubricant, degree of lubrication, platen material and
platen surface finish is consistent, especially when comparing the
engineering mechanical properties of breast implants (or other
elastomeric devices) from different manufacturers.
[0112] It is further contemplated that elastomeric devices having
substantially spherical geometry, cylindrical geometry, elliptical
geometry or combinations thereof can be described geometrically in
terms of the plate spacing of a load frame apparatus when
compressed, thereby allowing this invention to be applied to
compute engineering stresses, engineering strains and engineering
moduli from a load frame apparatus load-displacement data.
[0113] The elastomeric devices and breast implants that can be
tested using the methods of this invention are comprised of
materials including, but not limited to, silicone, polyurethane,
polyester, polyether, polystyrene, neoprene, polyisoprene,
polypropylene oxide, natural rubber, hydrogels, and composites or
co-polymers thereof. For medical devices such as breast implants,
the preferred materials should be biocompatible.
[0114] Although this invention has been described in specific
detail with reference to the enclosed detailed description and
invention embodiments, it will be understood that many variants,
modifications and combinations of the invention embodiments may be
effected within the spirit and scope of the invention as described
in the appended claims.
* * * * *